1. Field of the Invention
This invention relates to a method of producing isotopically enriched helium-4 (helium-4, the helium-3 content of which is below that of natural gas source helium) from liquefied natural gas source helium, and to an improved process for cooling a nuclear reactor with helium through the use of isotopically enriched helium-4 as the coolant.
2. Brief Description of the Prior Art
There are two natural sources of helium which have been used for its production, namely, natural gas and air, and there are two naturally occurring helium isotopes, helium-4 and helium-3. The predominant isotope, helium-4, possesses an atomic weight of 4 and comprises all but about one part per million (ppm) or less, on a volume or mol basis, of natural source helium. This isotopic form of helium becomes a superfluid liquid (possesses zero frictional resistance) at temperatures below around 2.2.degree. K. (obeys Bose-Einstein quantum statistics), and as a gas is impervious to radiation and possesses a low cross-section for neutron capture. Helium-3, the lighter isotope, has an atomic weight of 3, is present in natural source helium to the extent of about one ppm or less, does not become superfluid at low temperatures (obeys Fermi-Dirac quantum statistics), and undergoes conversion in a strong radiation field, by absoroption of beta particles, to tritium, the heaviest isotope of hydrogen, with a half-life of about 121/4 years.
The isotopic ratio of helium-3 to helium-4 in natural source helium is about an order of magnitude higher in helium derived from the air (atmospheric source helium) than in helium derived from natural gas (natural gas source helium). In times past, helium has been extracted on occasions for needed purposes from the air, but today the exclusive economic source of helium is natural gas. In atmospheric source helium the helium-3 content is about 1.2 ppm, relatively independent of location. In natural gas source helium, the helium-3 content varies somewhat, ranging, according to some of the older measurements of Aldrich and Nier (Physical Review, Dec. 1, 1948, p. 1590), from about 0.05 to about 0.5 ppm, depending on the location of the natural gas. These results are imprecise, with an admitted relative error of about 10-30%.
Later and more accurate measurements of helium-3 in natural gas source helium were made using high resolution mass spectrometers. These showed the helium-3 content of samples of natural gas source helium to run between 0.17 and 0.23 ppm, with a measurement precision of about 0.01 ppm, an order of magnitude better than that of the older measurements of Aldrich and Nier. Still more recently, very accurate instrumentation has been developed and used by the United States Bureau of Mines [Report of Investigations 8119 (1976)] to measure the amount of helium-3 in helium-4, the measurement uncertainty being something less than 0.001 ppm, or 1 part per billion (ppb).
Until very recently the helium-3 content of atmospheric source or natural gas source helium was of academic or theoretical interest only. However, in the last few years, with the advent of nuclear power, one of the major nuclear reactor designs which has emerged is that of the high-temperature gas-cooled reactor (HTGR), developed in the United States by General Atomic Company. The HTGR design uses helium gas as the coolant, to abstract heat from the graphite nuclear core, which heat is then converted first into mechanical, and then electrical, energy. The HTGR design development has imparted considerable practical significance to the matter of the helium-3 content of helium, as will hereinafter be discussed.
At the present time, there are two HTGR plants operating in the United States. The earlier one is the Beach Bottom unit of Philadelphia Electric Company, a pilot facility of 40 Mw(e), and the more recent one is the 334 Mw(e) Fort St. Vrain reactor of Public Service Company of Colorado. Orders have been placed with General Atomic Company for a half-dozen or so additional plants, in the capacity range 770-1160 Mw(e), with an estimated value of $2-3 billion.
The HTGR possesses a helium coolant inventory averaging about 2.0 million standard cubic feet (MMscf) of helium per 1000 Mw(e). The ratio varies somewhat, ranging from about 2.6 for the smaller reactor sizes in the 350 Mw(e) class to about 1.4 for the largest reactor size of 1150 Mw(e). The helium circulates at an operating pressure of 350-700 psig, at a rate of about one-fourth of its inventory per hour, and there is made up for mechanical and other losses some 10% of the inventory per year.
The advantage of helium over other coolants (except in certain respects hydrogen) reposes in its low density and high heat capacity, with attendant lower circulating rates and power requirements; its ability to operate without thermal decomposition at very high temperatures (so far, up to 2400.degree. F.), yielding high reactor thermal efficiencies; its chemical inertness (because it is a noble gas) toward any substance or component of the circulating system with which it comes in contact; its stability or imperviousness to radiation--it itself is a product, as an alpha particle, of the process of radioactive disintegration; and its low cross-section for neutron absorption or capture, giving good reactor neutron efficiencies. Further, helium is readily purified to a high degree, so that the impurities normally present in it--nitrogen, neon, water, and hydrogen--are in the very low ppm range.
In some respects, as hereinabove mentioned, helium is not as advantageous a coolant in the HTGR as is hydrogen, for the reason that hydrogen possesses a higher heat capacity, a higher thermal conductivity, a lower viscosity, and a lower density, producing thereby an approximate 1/3 overall increase in heat transfer rate and a reduction in required pumping power of about 1/2. Hydrogen does possess some disadvantages, however, in that it cannot be purified as well, is combustible and consequently for safety reasons needs to be kept to less than about 6% by volume in air, tends to disassociate at very high temperatures into atomic hydrogen which diffuses through most steels, and tends also at very high temperatures (apparently beyond those of the current HTGR design) to react slowly with graphite to produce methane, counterbalancing which detrimental methanation effect, however, is its tendency to suppress the corrosion of graphite by the accidental inleak of water from the steam-generator side of the HTGR. On the basis of these considerations, I have determined that there is a mixture of a modest amount of hydrogen in helium which, as a coolant, in overall respects, is superior to helium or hydrogen alone.
Now that some operating experience has been acquired with the helium-cooled HTGR, it has been discovered that the helium-3 content of the helium circulant is a detrimental impurity, in that it undergoes nuclear modification in the high intensity radiation field of the reactor to produce radioactive tritium, which because of its radioactivity level needs to be removed continuously, along with radioactive fission products which from time to time escape from the core of the reactor into the helium coolant. These other radioactive contaminants of the helium coolant, are, in the main, heavy inert gases such as krypton and xenon, and are not too difficult to remove from the helium by the relatively simple purification process of charcoal adsorption. Tritium, on the other hand, a hydrogen isotope, is much lighter than the other radioactive contaminants and is not readily removed by physical adsorption onto charcoal; it must be oxidized and the tritium oxide in turn adsorbed, or it must be removed by reaction with porous titanium metal sponge, which sponge must then physically be replaced when exhausted. Since tritium and hydrogen are sister isotopes, any removal of the tritium inevitably brings about the removal of all of the hydrogen also present, and the duty of the purifier is fixed by the sum of the amounts of tritium and hydrogen in the coolant. Thus, the removal of the tritium, however accomplished, poses a significant radioactive solid waste disposal problem, because of its relatively high level of radioactivity and its long half-life, and increases, because of concomitant, although unnecessary, hydrogen removal, the duty of the purifier.
Upon recognition of the problem of helium-3 in helium circulant, operators of the newer HTGR plants have attempted to fill and operate their systems with helium containing the lowest possible amount of helium-3. Since, however, the only helium which has been available for any purpose, including nuclear reactors, is ordinary commercial helium derived from natural gas (natural gas source helium) containing, as earlier described, approximately 0.20 ppm of helium-3, these operators in their recent purchases of helium have specified sources containing helium-3 in the lower 0.17-0.18 ppm range, even though it would be desirable and advantageous for them to use helium containing helium-3 in amounts an order of magnitude below this, which low helium-3 content helium, however, simply does not exist in nature.
It can thus be seen how advantageous it would be to produce and have as an HTGR coolant isotopically enriched helium-4, or helium the helium-3 content of which is significantly lower than that in natural gas source helium, and preferably below about 0.05 ppm, the benefits in HTGR use being related directly to the degree of reduction of the helium-3 content below that of natural gas source helium.
I have discovered that isotopically enriched helium-4, non-existent in nature, can be produced from liquefied natural gas souce helium by distillation therefrom of its helium-3, and when used in place of natural gas source helium as a coolant for a high-temperature gas-cooled reactor, can bring about an improvement in the form of a more efficient (lower cost) and safer (lower radioactivity level) reactor operation.
The separation of helium-3 and helium-4 is in itself not new in the art, but as heretofore practiced has not produced isotopically enriched helium-4 from natural gas source helium under the conditions of separation of the present invention. The prior art separation methods are the following:
1. The source material is a mixture of 1-3 mol percent helium-3 in helium-4, argon, air, and traces of tritium, derived from U.S. Energy Research and Development Administration (ERDA) operations which produce tritium by nuclear bombardment of lithium-6 isotope, the tritium subsequently decaying to helium-3. The source material is separated for the express purpose of producing therefrom a relatively high purity helium-3, the separation process being one of gaseous thermal diffision (Chemical Engineering, Nov. 25, 1963, p. 64), and carried out at the Mound Laboratory of Monsanto Research Corporation at Miamisburg, Ohio, under contract with ERDA. The products are helium-3 of 99 plus percent purity and residue gas with a helium-3 content of the order of 0.01 percent (100 ppm).
A recent improvement by Mound Laboratory on the enrichment process, using the same source material, is the substitution for thermal diffision of low temperature distillation under vacuum (pressure 130 mm, overhead column temperature 0.93.degree. K., bottom temperature 2.80.degree. K.), as described by Wilkes (Advances in Cryogenic Engineering, Plenum Press, Vol. 16, (1970), p. 298).
2. Helium-3 enrichment is achieved starting with natural gas source helium. One of the methods reported is that of cryogenic gas centrifugation (Newgard et al., U.S. Pat. No. 3,251,542). Another depends on the superfluid properties of helium-4 below around 2.2.degree. K., which permit it to separate from helium-3 by selective passage through a superleak, the helium-3 being retained (Keller, Helium-3 and Helium-4, Plenum Press, (1969), p. 36). The helium-3 enrichment achieved per pass is of the order of 5 times, with consequent low yields of enriched helium-3 and insignificant denuding of helium-3 in the remaining helium-4. Successive passes of the helium-3 enriched product to improve the helium-3 purity are more difficult and less successful, because the lambda point (the temperature at which superfluidity is achieved) decreases with increasing helium-3 content.
Recent improvements in the superleak separation process have been made by Mezhov-Deglin (Cryogenics, August 1972, p. 311, translated from Pribory i Tekhnique Eksperimenta, No. 3, 1971, p. 217) and Faturos et al. (Cryogenics, March 1975, p. 147), yielding an isotopically enriched helium-4 in the former case of about 0.05 ppm of helium-3, and in the latter case of about 0.0004 ppm of helium-3.
3. Helium-3 enrichment to a high purity helium-3 product is achieved starting with about 0.01% helium-3 in helium-4, the residue gas from the earlier-described thermal diffusion process of Mound Laboratory. The helium-3 enrichment is accomplished by a combination of superleak filtration and vacuum distillation, in that order, in a common apparatus, as disclosed by McKinney et al. (U.S. Pat. No. 3,421,334).
4. The feed material for the separation is about 6 mol percent helium-3 in helium-4. It is the heavier of two immiscible liquid phases in the helium-3, helium-4 dilution refrigerator, used to produce deep refrigeration, in the vicinity of 0.01.degree. K. Essentially pure helium-3 is removed from the feed by evaporation or pumping at about 0.7.degree. K. under very high vacuum to regenerate the helium-3 refrigerant for recirculation.
5. The feed material is atmospheric source helium, and it undergoes a vapor-liquid separation at pressures up to 1 atmosphere (Fairbank et al. Physical Review, Vol. 71, pages 911-913, 1947). Fairbank has determined the coexistent vapor-liquid phase equilibria and relative volatilities for helium-3 in helium-4 in this feedstock and in this pressure range. The helium-3/helium-4 relative volatility in the feedstock diminishes from something in excess of 5 near the lambda point to something below 2 in the vicinity of the normal boiling point of helium-4. Fairbank asserts an expectation--unsupported by experimental data--that the relative volatility of this system would become unity at the critical temperature of helium-4, namely 5.2.degree. K., at which point separation by distillation would be deemed impossible. The precise trend, however, of relative volatility from 4.2.degree. K., the normal boiling point of helium-4, to 5.2.degree. K., the critical temperature of helium-4, is unknown, even for Fairbank's atmospheric source helium feedstock, and it certainly is unknown over the entire pressure range for natural gas source helium, with its significantly lower initial helium-3 liquid concentration. It is well recognized that the relative volatility of the components of such a non-ideal system as this one, at these very low temperatures near absolute zero, is uncertain and unpredictable, and is strongly dependent on liquid concentration and system temperature and pressure.
Thus, so far as is known to me, isotopically enriched helium-4 has not heretofore been produced by separating helium-3 by distillation from liquefied natural gas source helium, and has not heretofore been used in a high temperature gas cooled nuclear reactor to improve the performance of the helium coolant in said reactor.
It was an unanticipated finding, therefore, that the separation by distillation of helium-3 from liquefied natural gas source helium to yield isotopically enriched helium-4 could take place at pressures and temperatures ranging from those in the vicinity of the lambda point (40 mm and 2.2.degree. K.) to those near the critical temperature of helium-4 (1700 mm and 5.2.degree. K.), and that said isotopically enriched helium-4, when used as a high-temperature gas-cooled nuclear reactor coolant, enabled a build-up of hydrogen in the coolent, resulting in a more economical, improved and safe nuclear reactor operation.